Leaf tea may be prepared as green leaf tea or black leaf tea. Generally, to prepare black leaf tea fresh green leaves of the plant Camellia sinensis are withered (subjected to mild drying), comminuted, fermented (in which process enzymes in the tea leaf use atmospheric oxygen to oxidise various substrates to produce brown-coloured products) and then fired (to dry the tea leaves). Green leaf tea is not exposed to the fermentation process. Partial fermentation may be used to produce intermediate-type teas known as “oolong” tea.
Conventional wisdom dictates that in black tea manufacture tea must be macerated in some way to liberate the fermentative enzymes and their substrates within the leaves. One can macerate tea in many ways but broadly speaking there are two main mechanised methods for doing this.
The first, called “orthodox manufacture”, involves rolling pre-weighed batches of heavily withered tea leaves prior to fermenting, firing and drying steps. So called “orthodox tea” is typically characterised by leaf particles that are aesthetically pleasing to many (resemble dried leaves rather than “granules” of CTC tea—see below) but produce lighter liquors due to less extensive fermentation and poorer infusion from the larger particles.
The second method is the most popular of a number of non-orthodox, continuous methods that involves using a machine resembling a mangle that cuts, tears and curls tea leaves. The original machine was invented by W. McKercher in 1930 and is commonly referred to as a CTC (cut-tear-curl) machine. The finely cut product is known generically as “CTC tea” and is characterised by a fast infusion rate and strong colour. This method tends to produce tea that is more consistent and uniform in quality and than the orthodox method but, as the CTC process tends to cause agglomeration of the tea material, it has the appearance of “granules” rather than leaf fragments.
Both orthodox and CTC rollers are often used in conjunction with a rotorvane machine, which is a type of extruder used to pre-condition (squeeze/shred) leaves prior to maceration. The rotorvane was invented as a form of continuous orthodox roller, but is rarely used as such today. Liquors generated from rotorvane teas are similar to orthodox in terms of liquor properties and infusion rates. These methods and their history and role in tea manufacture are described in “Tea: Cultivation to Consumption” edited by K. C. Willson and M. N. Clifford, Chapman & Hall, 1992.
Generally speaking consumer preference for either orthodox or CTC tea is a matter of national or regional culture. In many countries the visual appearance and texture of leaf tea are important indicators of quality, larger leaf particles being associated with higher quality. In Western markets tea is increasingly purchased in filter paper bags and the colour of the infused product tends to be more important.
Some consumers however want the best of both worlds; i.e. a leaf tea that looks and feels like orthodox processed tea but has the liquor characteristics of a fuller fermented CTC processed tea. The present applicants have developed methods for manufacturing black leaf teas that resemble orthodox processed tea but infuse like CTC processed tea.
Our international patent application WO 99/40799 discloses a method that involves subjecting whole tea leaves to a heat shock at a temperature and for a duration that is sufficient to initiate fermentation, and enabling the tea to ferment for a time and at a temperature that is sufficient to achieve desired liquor properties.
Our international patent application WO 00/10401 discloses a method for manufacturing fast infusing whole or large leaf teas by impregnating tea leaves with liquid carbon dioxide within a pressure vessel, depressurising the vessel at a rate that is sufficient to freeze the liquid carbon dioxide, applying sufficient heat to cause the frozen carbon dioxide to sublime and consequently initiate fermentation within the leaves, allowing the tea to ferment for a time that is sufficient to achieve desired liquor properties, and drying the fermented product to yield the whole leaf tea.
Our international patent application WO 01/11979 describes and claims a black leaf tea that has the appearance of orthodox processed black tea but is characterised in that it has the infusion characteristics of CTC processed black tea.
More recently our co-pending international patent application WO 01/82713 (claiming priority from United Kingdom patent application GB 0010315.0) discloses a method that involves withering a first supply of freshly plucked tea leaves, macerating the withered leaves, allowing the macerated withered leaves to ferment to produce fermented dhool, withering a second supply of freshly plucked tea leaves, mixing the fermented dhool obtained from the first supply of leaves with the withered leaves obtained from the second supply of leaves, rolling the mixture, allowing the rolled mixture to ferment, and drying the fermented mixture to yield black leaf tea.
Russian patent specification SU 1678276 discloses a method for processing tea leaves into tea granules and tea concentrates. Tea leaves are withered to a residual moisture content of about 64 to 65%, then rolled and graded. The smaller graded fraction is fermented and dried. The large fraction is cut and rolled then graded again. The small tea grades are processed further to make tea granules and the large grades are used to make tea concentrates. This is a modification of orthodox processing intended to increase the yield of high quality orthodox tea. The leaf products of this disclosure are traditional orthodox teas, i.e. slow infusing, high flavour, low colour teas.
Russian patent specification SU 1034686 discloses a method for manufacturing black leaf tea with good quality in a shorter than usual time. Tea leaves are withered to a residual moisture content of 70% moisture then rolled and graded. Larger leaves are reduced in size. The tea is mixed and simultaneously aerated with atmospheric air at 45 m3/hour for 15 minutes after which it is dry. This gives orthodox leaf i.e. slow infusing, high flavour, low colour teas.
The present inventors, through a deeper understanding of the fermentation and infusion processes of tea, have developed an alternative method for making fast-infusing larger leaf black tea. This method can utilise adapted tea manufacturing equipment (such as the rotorvane and CTC) or novel processing equipment, but requires a thorough understanding and control of raw material (green leaf) properties, and the interaction with the processing equipment.